The present invention relates to an image projection apparatus for projecting television images, and more particularly to an image projection apparatus for projecting color images by combining images generated from a plurality of cathode ray tubes.
In the technical field of image display, direct-viewing type cathode ray tubes (CRTs) for televisions have proven incapable of providing a relatively large image display, currently being limited to forty-inch screens. Thus, larger screen sizes generally bring about problems with respect to practicality, due to increased difficulty in manufacturing and diminishing resolution. On the other hand, since the image projection apparatus projects an image from image generating surfaces of small projection cathode ray tubes to a screen by means of an optical apparatus such as a lens, a large-sized image of better picture quality can be realized by improving performance of the optical apparatus.
The image projection apparatus generally comprises three projection-type CRTs each having an image generating surface with phosphors for illuminating red, green and blue, and projection lenses for enlarging and projecting images. Additionally, in order to combine the images of the three CRTs, a certain image projection apparatus comprises two dichroic mirrors which reflect one image of red, green and blue but allow other images to be passed (see U.S. Pat. Nos. 4,607,280 and 4,842,394).
Such an image projecting apparatus having two dichroic mirrors is constructed as shown in FIG. 1. Three CRTs 1, 2 and 3 are arranged in a T-configuration, and two dichroic mirrors 5 and 6 are arranged in an X-configuration such that respective dichroic mirrors are inclined by 45.degree. with respect to optical axes of the cathode ray tubes to intersect with each other at the point where the respective optical axes of the cathode ray tubes meet, and a projection lens group 7 is provided in front of the dichroic mirrors. In the image projection apparatus constructed as above, in order to enhance the performance of the optical system, an increase in the reflectance of the dichroic mirror is necessary along with the elimination of lens element aberration.
In U.S. Pat. No. 4,607,280, two dichroic mirrors are installed in a space filled with air. In such an arrangement, since coma aberration occurs due to the refractive index of the two dichroic mirrors, many lens elements are required for compensating the coma aberration.
In case of U.S. Pat. No. 4,842,394, the dichroic mirrors are installed in an oil-filled space having a predetermined refractive index, thereby contriving the improvement of axial performance. However, in such a structure, the refractive index of the oil is varied due to the heat generated in the cathode ray tubes, and the degree of shift in the so-called "stop band" according to the variance of incident angle of light emitted from the cathode ray tube toward the dichroic mirror is increased, resulting in a severe variation in color and a significant degradation of the reflectance.